1. Field of the invention
The present invention relates in general to forming of semiconductor package lead frame, and more particularly to a process and apparatus for progressively forming a lead frame of a quad flat semiconductor package having fine pitch leads.
2. Description of the Prior Art
As well known to those skilled in the art, the production method of semiconductor packages generally includes a progressive forming process for providing individual semiconductor packages having a plurality of leads bent into a desired shape using a forming punch die.
With reference to FIGS. 1A, 1B and 2, a lead frame 1 holding a plurality of quad flat semiconductor packages 2 having fine pitch leads 3 progressively passes through a forming punch die 4 as shown at the arrow of FIGS. 1A and 1B and subjected to a progressive forming process. When the lead frame 1 holding the plurality of semiconductor packages 2 reaches the forming punch die 4, a trimming step is carried out for cutting dam bars of the lead frame 1 and followed by a progressive forming step. In the progressive forming step, the plurality of leads 3 of each of the packages 2 are progressively bent into a desired shape. At the last position of the forming punch die 4, individual semiconductor packages 2 having the bent leads 3 are cut free from the lead frame 1.
The conventional forming punch die 4 is equipped with an integrated type forming punch or a cam type forming punch. Otherwise stated, as shown in FIG. 2, a forming punch 5 of the cam type is provided at the lower section of the forming punch die 4. While the lead frame 1 progressively passes through the forming punch die 4, the cam type forming punch 5 performs a repeated vertical reciprocation with respect to the lead frame 1 and carries out the progressive forming for the leads 3, thereby bending the leads 3 into the desired shape. Here, the forming punch 5 reciprocates at a velocity higher than 50 SPM., so that there is generated a face frictional heat of a higher temperature between the contact surface of the reciprocating forming punch 5 and the leads 3.
The temperature of the face frictional heat generated between the forming punch 5 and the leads 3 is higher than a plating temperature of the plated lead frame 1, so that the plating material, such as tin, of the lead frame 1 is melted and builds up on the contact surface of the forming punch 5 or between the leads 3 having the fine pitch of about 0.2 mm to 0.3 mm.
The build-up of the plating material on the contact surface of the forming punch 5 causes a scratch on the leads 3, thereby deteriorating the quality of the resulting semiconductor package. On the other hand, when the plating material builds up between the leads 3 having a pitch ranging ranged from about 0.2 mm to 0.3 mm, these leads 3 are often electrically connected to each other through the build-up material causing a short circuit of the resulting semiconductor package. This results in deterioration of the reliability and the productivity of the semiconductor packages.
In addition, there has been proposed a progressive roll forming of a semiconductor package carried out using a plurality of roller sets, such as disclosed in U.S. Pat. No. 5,074,139. This U.S. patent discloses a method by which leads of a semiconductor package are bent into their final shape by employing a roll forming process. This roll forming process achieves a final form by passing a lead frame holding a plurality of semiconductor packages through a series of form rollers which progressively bend the leads into the final shape. However, this forming method has a problem in that it is not used for forming the leads of a quad flat semiconductor package (QFP) but is used for forming the leads of a dual inline semiconductor package (DIP).